In many electronic applications wherein doppler radar and the like are utilized, the relative speed between the vehicle carrying the radar and an object of interest may be varying, such as when the vehicle carrying the radar passes close by the object of interest. In many applications it is desirable to provide an indication when the relative speed, or Doppler frequency shift, reaches a predetermined value. Also, it is desirable to have the ability to differentiate between signals returned from the object of interest and signals returned from, or emanating from, other objects. By knowing the approximate speed of the vehicle carrying the radar (including any alterations in the speed thereof) and the approximate speed of the object of interest, a general (or acceptable) range of approximate Doppler frequency shifts can be determined. Since this range of Doppler frequency shifts will be continually changing as the relative speed and relative position of the vehicle and object of interest changes, it is necessary to calculate, or chart, a continuously changing range of frequencies with time. When a return signal falls within the continuously changing range of frequencies, it is generally a return signal from the object of interest.
In the prior art, analog circuitry is generally utilized to provide a precalculated frequency pattern which may serve as an upper, or lower, limit for return signals. The analog circuitry is undesirable because it is extremely expensive and generally bulky.